X-ray diffraction and conductivity investigations of lanthanum-doped barium titanate ceramics

Grain size, lattice parameters, internal stress, conductivity, and permittivity of BaTiO3 ceramics doped with 0 to 5 at% La are studied. Up to 0.25 at% La the material is semiconducting and shows a pronounced PTCeffect (positive temperature coefficient of resistivity). Grain size, permittivity maximum, and conductivity exhibit a maximum at 0.2 at% and a minimum at 2 at% La. Within a rather narrow concentration range between 0.2 and 0.5 at% La the grain size decreases from more than 100 μm to 1 to 2 μm leading to the appearance of internal stress which results from the cubic to tetragonal phase transition. The decrease of the PTC effect observed in the same concentration range can be ascribed to a grain size dependent broadening of the phase transition region. The correspondence of grain size, internal stress, and phase transition behaviour suggests an internal stress induced diffuse transition for BaTiO3 containing more than 0.25 at% La.